Pouring nozzle for continuous casting liquid metal or ordinary steel



.N. F. TISDALE ET POURING NO Aprll 1, 1969 I 3,435,992

ZZLE FOR [commuous CASTING. I LIQUID METAL on ORDINARY STEEL Filed March11', 1966 20 Fig.l."

Fig.4.

INVENTORS NORMAN F. TISDALE ROM/LAND A.TI$DALE gTORNE Y United StatesPatent US. Cl. 222-146 8 Claims ABSTRACT OF THE DISCLOSURE Incombination with a ladle having a bottom opening, a refractory nozzlehaving an induction coil surrounding it and a source of high frequencycurrent for energizing the coil so as to inductively heat the interiorsurface of the coil to the temperature of molten metal being poured fromthe ladle.

In the casting of iron and steel by the continuous casting process,certain parts of the operations cause considerable difliculty. In orderto maintain a uniform flow of metal to the mold, a steady ferrostaticpressure is highly desirable to insure the free movement of the caststeel away from the mold and produce a good surface with uniformdimensions.

Nozzles are a source of trouble. Their apertures are small, erodequickly during pouring or build up by silica, or nonmetallic alumina,adherence at the hole. This changes the size of the stream and,therefore, the rate of flow. This, in turn, affects the movement of thesteel through the mold. Since the temperature of the liquid steel isusually high, it has an adverse effect on the ability of the nozzle togive good service.

An object of our invention is to provide a novel nozzle assembly whichovercomes the above named disadvantages of conventional nozzles such aspresently used in continuous casting systems or regular pouring of hotmetals so as to assure constant and even flow of molten metal throughthe nozzle at all times and thus assure a more perfect billet, ingot orslab.

A more specific object of our invention is to provide a novel nozzleassembly including an induction heated insert so constructed andarranged that the skin or hole-defining surface, through which the metalis poured, will be kept at sufficiently high temperature, that is, aboutor even higher than the temperature of the molten stream, so as toassure that metal or non-metallics will not solidify or freeze so as toconstrict the size of the opening through the nozzle.

Other objects and advantages of the invention will become more apparentfrom a study of the following description taken with the accompanyingdrawing wherein:

FIG. 1 is a schematic illustration of the tundish and ladle assembly ofa continuous casting system;

FIG. 2 is an enlarged, elevational View of insert 22 of the nozzle 34shown in FIG. 3, two of which are shown at the bottom of the tundish 16of FIG. 1;

FIG. 3 is a less enlarged, vertical cross-sectional view of a nozzleassembly including the insert 22 of FIG. 2 and an electrical, inductionheating circuit for heating the insert of the nozzle;

FIG. 4 is a top view of the nozzle shown in FIG. 3; and,

FIG. 5 is a schematic diagram of a modification of the heating circuitembodying resistance heating.

The casting of hot metal through a nozzle into a mold, receptacle ofsome other container, presents some problems. If the stream of liquidmetal going through the nozzle becomes lower in temperature andapproaches or reaches its solidifying point, the nozzle aperture canpartially or fully be clogged by such metal and stop the flow of metal.If such liquid metal contains material, foreign to its composition, thismay solidify earlier than the parent metal, here again, somesolidification may occur and thus block the nozzle aperture or reduceits flow. Our invention is effective to alleviate these conditions andallow the pouring of such liquid as desired.

Nozzles are usually made from a ceramic composition. They vary indesign, hole size and composition. They may erode quickly due to theflow of the liquid metal which reduces their life.

This is true in some special steels which are not continuously cast,such as silicon, stainless, heat resisting or high aluminum steels andin continuous casting.

Referring particularly to continuous casting of steel, obtaining a goodsurface of the billet, bloom or slab is greatly dependent on maintainingan even, constant ferrostatic pressure on the mold. When the pressurevaries, it affects the complete filling of the mold and results in crosssections that are of unequal dimensions. It may even result in a centerwhich may be porous or a large secondary pipe may be formed. Thus can beseen the necessity of maintaining an even pouring rate and to keep thenozzle properly functioning at all times. A further risk occurs whenthere are more than one strand operating, if one or more nozzles becomeout of order it puts an extra burden on the remaining strands and thetemperature of the metal in the tundish may not be high enough tomaintain the fluidity of the remaining metal to complete its fullpouring.

Again, when the pouring temperature of steels falls below approximately2900 degrees F, a film of non-metallics with or without iron begins toform in the nozzle and gradually builds up until the nozzle may becompletely or nearly completely closed, thus shutting down that part ofthe equipment.

This non-metallic buildup causes a restriction on the amount of aluminumthat can be used, usually .4 lb. maximum of aluminum per ton. Thisrestricts the use of continuous casting to certain grades of steel.Steels such as deep drawing sheet steels which are made in largetonnages in the regular manner and must use in excess of 2 pounds ofaluminum per ton of steel may not be made by the continuous processusing the old type nozzles.

Our heated nozzles overcome most of the clogged nozzle troubles.

We propose to have built into the nozzle, an insert which will carry anelectrical current. This insert Will be part of the nozzle and the partof the insert that is exposed to the fiow of metal may be coated orplated with a high temperature heat resisting material or ceramic toprevent wear or rapid erosion. The temperature of the area in thenozzle, where clogging usually occurs, Will be maintained above themelting point of the clogging material thus keeping the nozzle clear foruninterrupted pouring.

Referring more particularly to FIG. 1, numeral 10 generally denotes theinitial section of a continuous casting system, embodying a ladle 12having a stopper 13 which opens and closes a hole 14 at the bottomthereof to allow molten metal to pour into a tundish 16. The tundish hasholes (two being shown) at the bottom thereof fitted with refractorynozzles through which molten metal is poured into two oscillating,water-cooled molds to form a pair of strands of cast steel .20, 20'.These strands, in a known manner, are then conducted to pinch rolls,billet bending rolls, discharge chutes for changing the direction of thestrand from vertical to horizontal, a billet straightener, and anautomatic cutting machine for cutting the billets into lengths which arethen stacked.

Referring more particularly to FIGS. 2, 3 and 4, numeral 22 generallydenoted a nozzle insert which is closely fitted into a ceramic nozzle 34of zirconium oxide or other suitable substantially nonelectricallyconductive refractory material. The insert comprises a conductivematerial, such as portion 28 of graphite, which forms a close and tightfit inside the correspondingly shaped, well portion formed in nozzle 34.The insert 22, or at least the refractory plug 38 may be replaced fromtime to time without the necessity of replacing the induction coil 42.

In order to prevent erosion of the graphite portion 28 by the moltenstream a hard surface coating 32 of about .02 to about .03 inch thick isprovided on the surface of portion 28 which defines the aperture whichis in registry with aperture of plug or nozzle 38. This hard surface orcoating 32 may be of silicon carbide, tungsten carbide, zirconium oxide,zirconium carbide, boron carbide, yttrium boride or oxide or similarmaterials that will resist erosion by a molten stream of about 2,950degrees F. It was found that the abovementioned difiiculties of cloggingcould be averted by raising the temperature of the bore surface of thenozzle by about 100 F. above the casting temperature or the meltingpoint of the clogging material.

In order to heat the carbon insert portion 28 and coating 32 to at leasta temperature of about 2900 F. or perhaps higher than the molten streamtemperature of about 2950 R, we have found that the most efficient modeis induction heating which is done by a helical induction coil 42closely surrounding the ceramic nozzle or plug 34, which coil 42 isenergized by a motor generator 60 through a pair of line conductors 44,46, across which are connected a fixed condenser 48 and a plurality ofvariable condensers 50, 52, 54, 56, and 58 which are inserted in thecircuit as the temperature increases since electrical properties changewith the temperature. Such condensers are used to compensate forvariations occurring before and during operation of the continuouscasting system or regular liquid metal pouring.

A unique feature of the invention is that the induction heating will notappreciably raise the temperature of the ceramic nozzle 38, but willconsiderably increase the temperature of the graphite liner or insert22, particularly the coated surface 32, Where the high temperature ismostly needed so as to prevent congealing or solidification of metalfrom the molten stream passing through apertures 30 and 40. Theprinciples of induction heating as contrasted with other means ofheating is summarized in Chemical Engineers Handbook, 4th edition, pp.2541 to 2543, by John H. Pen'y, McGraw-Hill Book Company. The coatedsurface 32 may be kept perhaps below, or higher, or at the sametemperature as that of the molten stream.

If another suitable coating bonding for the graphite insert withsufficiently long life is desired, the insert itself may be of othermaterials than graphite, such as zirconium carbide ZrC or zirconiumdiboride ZrB2 or Yttria zerconia.

It should be especially noted that for the highest efficiency, inductionheating frequencies may range from about 7 to 50 kilocycles. For smallhole diameters of about /2 inch, 3 to 7 kilocycles may be suitable, butfor holes of the order of 1 /2 inch and up, the frequency may be in therange of 40 to 50 kc. For extremely high frequencies, such as 15 to 50kc. a mercury gap oscillator may be employed.

In operation the tundish 16 is preheated in the regular manner and themetal is allowed to flow in from the ladle 12. The current is turned onin advance to the nozzle 18 or the surface exposed to the liquid steeland its temperature raised to 2900 degrees F. or higher, as desired.This temperature is maintained throughout the pour.

The present induction heating system may also be modified so as to heatthe molten stream instead. However, this extra electrical loading by thestream and rapid disposal of the heat by the stream involvessubstantially greater power consumption than the use of induction 4heating, which induction heating provides eddy currents for heating thesurface of the carbon portion 28 and coating 32.

FIG. 5 shows a modification involving a source of A.C., for energizingthe primary of a transformer 68 after closing of switch 82 of theprimary circuit, whereby the secondary coil passes current through afixed resistor 64 and a variable resistor 66, which resistors may beembedded in an insulating insert substituted for the insert 22 of FIG.3. However, resistance heating, as shown in FIG. 5, is not veryeflicient as compared to the induction heating system shown in FIG. 3.

A further modification could be capacitance or dielectric heating,involving embedding a pair of electrodes in an insulating insert, shapedlike insert 22, and applying a source of potential across suchelectrodes. However, such capacitance heating is likewise not asefficient as the induction heating system shown in FIG. 3.

While the novel nozzle assembly embodying the present invention has beendescribed in connection with a continuous casting system for improvingthe flow of molten metal, it has also other general applications, suchas in the ladle 12 or in any other ladies or containers wherein it isdesired to make sure that solidification will not clog the outlet holethereof when a liquid metal is to be poured through a nozzle.

While induction coil 42 is shown as surrounding the ceramic nozzle 38,it could be positioned in an annular well portion formed therein,instead, for additional pro tection.

Thus it will be seen that we have provided an efficient nozzle assemblyembodying a ceramic nozzle having an electrically conductive insertwhich is inductively heated at the surface thereof to very hightemperatures, approximating or exceeding the temperature of the moltensteel poured therethrough so as to prevent solidification or congealingof metal and clogging the hole opening.

While we have illustrated and described several embodiments of ourinvention, it will be understood that these are by way of illustrationonly, and that various changes and modifications may be made within thecontemplation of our invention and within the scope of the followingclaims.

We claim:

1. In combination with a container having at the bottom thereof a nozzlecomposed of a substantially nonelectrically conductive refractorymaterial through which molten metal is poured, an induction coilsurrounding and substantially coaxially disposed with the nozzleopening, an electrically conductive insert surrounding the nozzleopening and fitting into a well portion formed in the opening of saidnozzle, and a source of high frequency current in the range of betweenabout 3 to 50 kc. for energizing said coil so as to inductively heat theinsert in said nozzle to substantially the temperature of said pouredmolten metal.

2. The combination recited in claim 1 wherein said refractory nozzle isformed from a material selected from the group consisting of zirconiumoxide, zirconium carbide and zirconium diboride.

3. The combination recited in claim 1 wherein a graphite insertsurrounds the nozzle opening.

4. The combination recited in claim 3 wherein a coating of a materialselected from the group consisting of zirconium oxide, zirconiumcarbide, boron carbide and yttrium boride is provided on thehole-defining portion of the nozzle to protect the graphite fromerosion.

5. The combination recited in claim 4 wherein said graphite insert isprovided with a refractory protective coating to prevent erosion by themolten metal.

6. The combination recited in claim 3 wherein fixed and variablecondensers are connected in parallel with said induction coil.

7. A method for preventing the congelation of molten metal in passingfrom a receptacle through a substantially nonelectrically conductivenozzle orifice to maintain uniform flow thereof, comprising lining theorifice with an electrically conductive insert, applying a protectiverefractory coating to the insert, disposing an induction coil about saidnozzle, energizing the coil with a high frequency current to inductivelyheat the insert to substantially the temperature of the molten metal.

8. That method of assuring maintenance of constant ferrostatic pressureon a mold in the continuous casting of steel by preventingsolidification of metal in a substantially nonelectrically conductivenozzle used in said continuous casting comprising, installing in saidnozzle a refractory electrically conductive insert having asubstantially centrally located passageway, there being an inductioncoil about said nozzle, energizing said coil with a high frequencycurrent suflicient to maintain the insert defining said passage atsubstantially the temperature of the steel to thereby prevent theformation of a film and the gradual building up thereof in said passagewhich would close said passage thereby assuring a constant ferrostaticpressure while said molten metal is flowing through said passage.

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US. Cl. X.R. 2.2,276, 566; 26642; 164-281

